You are correct. It relates to the torque (rotational force) that can be produced at different RPM.

Diesel engines have a narrow range of RPM where the torque is highest. At low RPM, the torque of a diesel engine is low. That’s why trucks have to shift into a low gear to start moving and shift multiple times to get up to speed. The driver uses the different gears to keep the engine running within the optimal range of RPM.

Electric motors have high torque at low RPM, so they can start a heavy load moving without having to have any complicated gearbox between the motors and drive wheels.

In a diesel-electric locomotive, the diesel generator can be designed to run at an efficient speed without being affected by the mass of the train. The electricity it generates powers the electric motors which have the necessary torque to handle moving the train.

You now have to lug around a LOT of both fuel and water, instead of just water and dry coal. Water and oil are both heavy by comparison to coal when lugging a train car of it around.

I think you’re making an assumption there. You would need to consider energy density of the fuel. Diesel fuel has almost twice the energy density of coal. For the same trip, the weight of the diesel fuel you would need for an oil-fired steam engine would be just a little more than half the weight of coal needed for a coal-fired steam engine.

Also, delivering the diesel fuel to the boiler would require a less complex mechanism and/or less workers than coal.